Fix typo "effiency" to "efficiency" in 3.4 - 3.8.

The quote in the beginning of the code coverage documentation is wrong.

Source: "Beautiful Testing"
http://shop.oreilly.com/product/9780596159825.do

Chapter 11 written by Murali Nandigama starts with:
> **The beauty of testing is found not in the effort** but in the efficiency.
Knowing what should be tested is beautiful, and knowing what is being tested is beautiful.
Merge pull request #161 from hkdobrev/code-coverage-quote

Fix typo "effiency" to "efficiency" in 3.4 - 3.8.